This application claims the priority of German application 197 41 867.8-21, filed in Germany on Sep. 27, 1997, the disclosure of which is expressly incorporated by reference herein.
The invention relates to a brake actuator, comprising an electric motor, a conversion device for converting the rotational movement of the electric motor into a linear movement, and a hydraulic transmission for transmitting the linear movement of the conversion device to an adjusting piston of a brake.
Increasingly dense road traffic requires systems which, on the one hand, contribute to the driver's safety and, on the other hand, relieve the driver. In the automobile industry, the trend is in the direction of so-called autonomous driving.
In the future, the driver is to be aided by so-called "by-wire systems" and, in many driving situations, is to be relieved by intelligent systems. An important prerequisite of such systems is their electric controllability.
In the case of known brake systems, predominantly hydraulic or pneumatic transmission or booster devices are used by means of which the driver of a vehicle directly defines the brake intervention of a wheel brake. In order to permit, in systems of this type, driver-independent braking interventions, for example, for controlling a ranging, or the like, in addition to the necessary pressure supply by means of a relatively high pressure, high expenditures of components are required, for example, with respect to valves and the like.
For this reason, electrically operable wheel brakes have been known for some time which permit not only a direct controlling of the brake but, particularly compared to hydraulic or pneumatic braking systems, also require significantly fewer component parts.
From German Patent Document 195 19 308 A1, a brake actuator with a transmission is known, for example, which brake actuator has an electric motor as a driving element, a conversion device for converting the rotational movement of the electric motor into a linear movement and a hydraulic transmission for transmitting the linear movement of the conversion device to a friction lining of a brake.
In the case of the hydraulic transmission, this is implemented by means of a conversion device constructed as a spindle in that the spindle nut is frictionally connected with the electric motor and the spindle rod is frictionally connected with a hydraulic piston of the hydraulic transmission and in that, for receiving the spindle stroke, the electric motor is equipped with a hollow shaft. In the case of this brake actuator, the hydraulic piston is arranged in a closed hydraulic chamber.
The advantage of this brake actuator is its compact shape, but it is a problem that, in the case of a self-locking of the transmission, a danger arises for the driver and the environment because the brake would remain in the applied position, which results in an uncontrollable danger situation.
In order to implement, in the case of an electromagnetic wheel brake, different adjusting speeds and application forces of a tensioning device by means of an electric motor, in the case of a wheel brake known from German Patent Document DE 43 12 524 A1, clutches are provided which can be switched electromagnetically. Also in the case of this wheel brake, an adjustment of the brake is achieved by means of a spindle drive. A self-locking of the transmission is also a problem in the case of this brake. Furthermore, such a brake requires a plurality of high-expenditure component parts which are often susceptible to disturbances.
It is therefore an object of the invention to further develop a brake actuator of the above-mentioned type in that, by means of components which are as insusceptible to disturbances as possible, on the one hand, an electrically controllable adjustment of a brake is permitted at different adjusting speeds and application forces, and, on the other hand, a self-locking of the transmission and/or of the conversion device is excluded.
In the case of a brake actuator of the initially mentioned type, this object is achieved according to the invention in that the conversion device comprises at least two adjusting elements which can be driven by the electric motor and by means of which the at least two hydraulic pistons of the hydraulic transmission can be operated independently of one another in a hydraulic chamber which, as required, can be connected with a reservoir.
By means of the at least two mutually independently operable hydraulic pistons of the hydraulic transmission, an adjustment of the brake is advantageously permitted at, on the one hand, different adjusting speeds and, on the other hand, different application forces. As the result of the fact that the hydraulic chamber, in which the hydraulic pistons move, can be connected with a reservoir, as required, at any time, particularly in danger situations which occur, for example, when a self-locking of the adjusting elements occurs and thus also of the hydraulic pistons, a moving back of the adjusting piston and thus an opening of the brake by connecting the hydraulic chamber with the reservoir is permitted.
In this case, when the adjusting piston is moved back, the hydraulic fluid situated in the hydraulic chamber expands into the reservoir.
However, inversely, by means of the connection of the hydraulic chamber with the reservoir, a compensation can also be carried out of a volume increase of the hydraulic chamber which takes place, for example, in the case of a brake disk wear or a brake lining wear.
Purely in principle, many different embodiments are conceivable for connecting the hydraulic chamber with the reservoir.
A particularly advantageous embodiment, which particularly also permits an electric controllability, provides that the hydraulic chamber can be connected with the reservoir by way of a valve which can be controlled independently of the electric motor, and the hydraulic chamber, the valve and the reservoir form a system which is closed with respect to the environment.
By controlling the valve for the connection of the hydraulic chamber with the reservoir independently of the electric motor, on the one hand, the above-described emergency operating function of the brake can be implemented particularly advantageously. Furthermore, by the opening of the controllable valve, in a simple manner, a compensation of the brake disk wear and of the brake lining wear can also be carried out in the above-described fashion. As the result of the fact that the hydraulic chamber, the valve and the reservoir form a system which is closed with respect to the environment and which is preferably provided with a life-long charge, a "dry" brake actuator is also permitted which is therefore particularly friendly with respect to the environment.
In order to increase the safety of the brake and particularly ensure that, for example, in the event of a failure of the electric motor, an opening of the brake is possible, it is advantageously provided that the electric motor and the valve have two separate electric circuits which can be controlled independently of one another. As a result, it is ensured that, for example, in the event of a failure of the electric circuit for the electric motor, an operating of the valve is nevertheless possible by way of its separate electric circuit.
As far as the construction of the adjusting element is concerned, many different embodiments are also conceivable here.
An embodiment, which technically can be implemented in a particularly simple manner by means of few component parts and is therefore very advantageous, provides that the adjusting elements assigned to the two hydraulic pistons are spindle drives which can be operated by two driving elements which can be driven in a mutually independent manner by the electric motor.
In order to permit particularly an extremely compact construction of the whole brake actuator and thus of the whole wheel brake, it is advantageously provided that the first driving element is a first disk which engages with an external spindle thread of the first hydraulic piston and has a central internal spindle thread and engages with an external toothing in a gear wheel arranged on the motor shaft of the electric motor. This externally toothed disk is therefore part of a step-down gear in the form of a spur gear which transmits a rotation of the motor shaft in a geared-down manner into a spindle drive which, in turn, results in an axial displacement of the first hydraulic piston.
In this case, it is advantageously provided that the second driving element is a second disk which is arranged in parallel to the first disk and which, on a cylindrical projection shaped to it, has an external spindle thread which engages with an internal spindle thread of the second hydraulic piston and can be driven by the first disk by means of a controllable electromagnetic friction clutch.
The construction of the driving elements as a first and second disk particularly permits an extremely compact construction. The controllable electromagnetic friction clutch permits in a simple manner an operation of the second hydraulic piston which is independent of the operation of the first hydraulic piston by means of a single electric motor. For this reason, an additional electric motor or another driving source for operating the second hydraulic piston can be eliminated.
In order to, in particular, also design the bearings of the first and the second disk as compactly as possible, it is provided in an advantageous embodiment that the first and the second disk are freely movably disposed on one another and in each case on one half of a housing accommodating the whole conversion device and the hydraulic transmission or on a guide of the hydraulic piston. As a result, the number of parts of the brake actuator is also significantly reduced because, on the one hand, the housing and, on the other hand, the guide of the hydraulic pistons are a component of the bearings so that additional bearing elements are not necessary.